1. Field of the Invention
The present invention relates to a liquid sealed semiconductor device in which a semiconductor element, such as a semiconductor acceleration sensor, for example, and liquid are hermetically sealed within a package thereof, and it also relates to a method of assembling the same.
2. Description of the Prior Art
A semiconductor acceleration sensor is explained as an example of a semiconductor device in which a semiconductor element and liquid are hermetically sealed within a package thereof.
A sensing part of a semiconductor acceleration sensor usually comprises a cantilever. If the cantilever resonates, the sensing part is damaged or responds to an unnecessary frequency range. In order to avoid the resonance, damping liquid is employed as a high cut-off filter. This semiconductor acceleration sensor is assembled through the steps of installing the sensing part in a base, joining a cap to the base by screws to form a package injecting damping water into the package and hermetically sealing the package.
When the semiconductor acceleration sensor is applied to detecting acceleration of a motor vehicle or the like, the ambient temperature varies in the range of -40.degree. C. to +120.degree. C. When damping liquid expands or contracts within the temperature range, damping liquid leaks out of the package through the joining gap between the base and the cap.
A highly airtight package is required in order to prevent a leak of damping liquid under a rigorous operating condition. Even such a highly airtight package, however, can not be free from being influenced by changes in the internal pressure within the package due to the expansion or the contraction of the damping liquid. When the damping liquid expands, the internal pressure increases. As a result, the package is deformed and the capacity thereof increases. On the other hand, when the damping liquid contracts, the internal pressure decreases and there is a vacuum portion formed within the package. The deformation of the package due to the expansion or the contraction of the damping liquid has an adverse effect on the sensing part. The deformation of the package can be prevented by improving rigidity and pressure resistance of the package. Manufacturing cost for such a highly rigid and pressure-tight package, however, increases significantly. Furthermore, the pressure of the damping liquid rises significantly at high temperature. As a result, the operation of the sensing part is greatly affected and detecting accuracy is deteriorated.
A conventional semiconductor acceleration sensor shown in FIG. 8 has been produced in order to solve these problems.
FIG. 8 is a vertical sectional view shematically illustrating the conventional semiconductor acceleration sensor. A base 1 is made of a metal or the like and has a welding part 1a formed at the peripheral portion. A cap 2 is made of a metal or the like and has a welding part 2a formed at the peripheral portion. The base 1 and the cap 2 are jointed together to form a package.
The cap 2 has an injection hole 2b and an air hole 2c at the top face as well as a bulkhead 2d at the inner wall of the top face. A pedestal 3 is adhered to the base 1. A cantilever 4 has a fixed edge which is adhered onto the pedestal 3 and a free edge which is allowed to move free. The pedestal 3, together with the cantilever 4, forms a sensing part.
A detector 5 is formed on the cantilever 4. A semiconductor strain gauge is formed in the detector 5 by a thermal diffusion method or an ion-implantation method. The semiconductor strain gauge is electrically connected with resistive elements, for example, through a wire which is formed by an aluminum deposition, for example, to thereby organize a full-bridge circuit.
A weight 6 is mounted to the free edge of the cantilever 4. A leading pin 7 passes through the base 1 and protrudes to the outside of the package. Hard glass 8 fastens the leading pin 7 to the base 1 and seals the clearance between the leading pin 7 and a through hole of the base 1. A wire 9 made of gold or aluminum wire-bonds the leading pin 7 and the detector 5. Silicon oil or the like is employed as the damping liquid 10 and is hermetically sealed within the package. A portion representing 20-30% of the whole capacity of the package does not contain the damping liquid so as to form an air layer 11. (The air layer 11 occupies the portion shown in FIG. 8 when the side A comes to the top.)
The process for sealing liquid and air within the package of the conventional semiconductor acceleration sensor will be described below. Before the sealing process is conducted, the pedestal 3 is adhered to the base 1, and the cantilever 4 is mounted to the pedestal 3. In the first step, the welding part 2a of the cap 2 is electric-resistance-welded to the welding part 1a of the base 1 so as to form the hermetically sealed package. Then, a certain amount of the damping liquid which corresponds to about 70-80% of the whole capacity of the package is injected into the package through the injection hole 2b. At the same time, the air within the package is discharged through the air hole 2c so that about 20-30% of the air is left within the package to thereby form the air layer 11.
Then the injection hole 2b and the air hole 2c are hermetically soldered so that the damping liquid 10 and the air layer 11 are hermetically sealed within the package.
The semiconductor acceleration sensor thus assembled is set so that the side A thereof comes to the top. Consequently, the air layer occupies the upper portion of the sensor. The sensor is mounted to a motor vehicle or the like with the side A always being a top face. When acceleration is applied to the free edge of the cantilever 4, resistance value of the semiconductor strain gauge changes so that unbalanced voltage occurs within the full-bridge circuit. The semiconductor acceleration sensor detects the acceleration on the basis of the value of the unbalanced voltage.
When the semiconductor acceleration sensor is mounted to a motor vehicle or the like, the ambient temperature changes in the range of -40.degree. C. to +120.degree. C. According to the temperature change, damping liquid expands or contracts. The expansion or the contraction, however, is absorbed by the air layer 11 remaining in the package. The increase or the decrease of the internal pressure within the package due to the expansion or the contraction of the damping liquid is thus absorbed by the air layer 11. Therefore, the damping liquid does not leak out of the package through the joining gap between the welding parts 1a and 2a, and the package can maintain a high airtightness. The bulkhead 2d provided at the inner wall of the package protects the cantilever 4 from being affected by waves of the damping liquid or air bubbles of the air layer which causes noise.
When the above-mentioned conventional semiconductor acceleration sensor is mounted, it is necessary that the side A always comes to the top, whereby the sensing part occupies a lower part of the package and the air layer 11 occupies an upper part thereof as shown in FIG. 8. Therefore, the conventional sensor needs to be mounted only in a confined direction. There is another problem with the conventional semiconductor acceleration sensor when it is mounted to a motor vehicle or the like. In some cases, as in the case where a motor vehicle to which the conventional sensor is installed leans towards one direction to a great degree, for example, some of the air in the air layer 1 may goes beyond the bulkhead 2d and enters in the sensing part which has been completely submerged in the damping liquid. In such cases, a damping effect is reduced or is extinguished by the air, and noise is liable to occur. There is another problem in the process of assembling the conventional semiconductor acceleration sensor. The process for assembling the conventional semiconductor acceleration sensor comprises three steps, i.e., a step of providing the bulkhead 2d at the inner wall of the cap 2, a step of forming the package by welding the base 1 and the cap 2, and a step of forming the air layer within the package and sealing the injection hole 2b and the air hole 2c. Such a complicated process causes a high manufacturing cost.